There is a national interest in the discovery of alternative sources of fuels and chemicals, other than from petroleum resources. As the public discussion concerning the availability of petroleum resources and the need for alternative sources continues, it is anticipated that future government mandates will require transportation fuels to include, at least in part, hydrocarbons derived from sources besides petroleum. As such, there is a need to develop alternative sources for hydrocarbons useful for producing fuels and chemicals.
One possible alternative source of hydrocarbons for producing fuels and chemicals is the natural carbon found in plants and animals, such as for example, in the form of carbohydrates. These so-called “natural” carbon resources (or renewable hydrocarbons) are widely available, and remain a target alternative source for the production of hydrocarbons. For example, it is known that carbohydrates and other sugar-based feedstocks can be used to produce ethanol, which has been used in gasohol and other energy applications. However, the use of ethanol in transportation fuels has not proven to be cost effective.
Carbohydrates, however, also can be used to produce fuel range hydrocarbons. The upgrading of biologically derived materials to materials useful in producing fuels is known in the art. However, many carbohydrates (e.g., starch) are undesirable as feed stocks due to the costs associated with converting them to a usable form. In addition, many carbohydrates are known to be “difficult” to convert due to their chemical structure, or the hydrocarbon product produced is undesirable or will result in low yields of desirable products. Among the compounds that are stated to be difficult to convert include compounds with low effective hydrogen to carbon ratios, including carbohydrates such as starches and sugars, carboxylic acids and anhydrides, lower glycols, glycerin and other polyols and short chain aldehydes.
There has been a significant effort to produce lower polyols through catalytic hydrogenolysis of aqueous sorbitol. Various Group VIII metal hydrotreating catalysts have been discussed including nickel (U.S. Pat. No. 4,338,472), ruthenium (U.S. Pat. Nos. 4,496,780, 6,291,725), and rhenium (U.S. Pat. Nos. 6,479,713, 6,841,085). Alditols including 15-40 wt % sorbitol solution in water are catalytically hydrocracked between 400° to 500° F. and hydrogen partial pressure from 1200 to 2000 pound per square inch gauge (psig) in a fixed bed catalytic reactor using nickel catalyst to produce at least 30 wt % conversion to glycerol and glycol products (U.S. Pat. No. 4,338,472). In U.S. Pat. No. 4,496,780 an alkali promoter such as calcium hydroxide or sodium hydroxide was added to the feedstream solution to control pH, prevent nickel leaching and enhance conversion. Sorbitol was hydrocracked over a supported Group VIII noble metal catalyst with an alkaline earth metal oxide; such ruthenium on a titanium alumina support with barium oxide between 300° to 480° F. at 500 to 5000 psig to produce lower polyols such as glycerol, ethylene glycol, 1,2-propanediol. High molecular weight polyols including sugar alcohols such as sorbitol or xylitol in water with a base promoter underwent hydrogenolysis over a metal catalyst of ruthenium deposited on an alumina, titania, or carbon support between 350° to 480° F. at 500 to 2000 psig hydrogen to produce low molecular weight polyols including glycerol, propylene glycol, and ethylene glycol (U.S. Pat. No. 6,291,725). Five carbon sugars and sugar alcohols including 15-40 wt % sorbitol, and lactic acid were hydrocracked with hydrogen over a rhenium catalyst in water to achieve at least 30 wt % conversion to glycerol and glycol products between 400° and 500° F., between 1200 and 2000 psig hydrogen, and a liquid hourly space velocity of 1.5 to 3.0 (U.S. Pat. No. 6,479,713). Battelle (2005) reacts an aqueous solution of sorbitol with hydrogen over a multi-metallic rhenium catalyst, including Re and Ni, at 250°-375° F. to produce propylene glycol through hydrogenolysis of C—O and C—C bonds (U.S. Pat. No. 6,841,085). These methods are limited by size, temperature, products, and conversion rates. Unfortunately at higher temperatures and higher catalytic activity, these reactions become quickly fouled. The catalyst must be removed and replaced before sufficient volumes of fuel are processed. Thus, these reactions must be improved to meet a commercial production scale and cost effectiveness.
However, these processes are often complex and costly, with reaction products produced during coking oftentimes undesirable. This results in low percentages of desired products, often increasing undesirable byproducts such as carbon monoxide and carbon dioxide. Additionally, the high sugar content and high temperatures of the conversion process introduce unique coking issues when converting carbohydrates to sugar alcohol (also known as a polyol, polyhydric alcohol, or polyalcohol) and gasoline boiling range hydrocarbons. Frequently, improving catalyst activity is difficult because not all improvements are additive, many can have unforeseen detrimental effects leading to increased byproduct formation, increased coke formation, decreased catalyst life, and an inability to regenerate fouled catalysts. As such, developing a carbohydrate conversion process resulting in a product with reduced byproducts such as carbon monoxide, carbon dioxide, and coke production, would be a significant contribution to the art.